When outside air is taken into factories, buildings, etc., it is passed through filter units with glass fiber filters (as shown in FIG. 1) being folded and housed in a frame (as shown in FIG. 2) so as to remove dust from the air. When the outside air is relatively dry, these filter units can remove salt particles which are in the state of crystalline solid particles, and other solid particles usually contained in the air. When the outside air is highly humid, however, salt particles once captured on the surface of the filter deliquesce, spread over the filter surfaces like a film, and cause a sudden increase in pressure loss. In time, the salt intrudes into factories, buildings, etc., and gives rise to salt damage. The same phenomenon occurs, too, when seawater particles from the surface of the sea are carried on and blown by a strong gale.
In recent years, there have been proposed various salt damage-preventing filters. One of those filters consists of a water-absorbent layer and a water-repellent layer, with the former located beneath or downstream of the latter (as shown in FIG. 3). This type of filter aims at preventing passage of deliquescent NaCl through the filter, first by the action of the water-repellent layer, and then by allowing the water-absorbent layer to absorb and hold the redundant NaCl that passed through the water-repellent layer. Another example has a structure wherein a space is held between two water-repellent filters (as shown in FIG. 4). With this filter unit, deliquescent NaCl is repelled first by a water-repellent filter located upstream, and then by a water-repellent filter located downstream when NaCl has passed through the upstream filter.
However, when filters are placed horizontally, the deliquescent NaCl liquid droplets stay on the filter surfaces for a long time and decrease the area usable for ventilation, thus increasing pressure loss. Even if the filters are used in the direction of gravitation, the deliquescent NaCl spreads on the surface of the upstream filter like a film and leads to an inceased pressure loss, promoted by the fact that water-absorbent particles other than NaCl adhere to the surface of the upstream filter in practical use. Such increase in the pressure loss induces passage of the deliquescent NaCl through the upstream layer onto the downstream layer.
When an upstream filter has water-repellency, the salt liquidized by deliquescing phenomenon, which passed through the upstream filter never returns to the upstream filter.
In the case of the filter as shown in FIG. 3, an increase in pressure loss for the reasons described above forces NaCl to be pushed out from an upstream filter toward a downstream water-absorbent layer and it is absorbed and retained there by said layer; in a long-time operation, however, scattering of the NaCl from the water-absorbent layer occurs.
In the case of the filter as shown in FIG. 4, the peculiar structure having two water-repellent layers does not satisfactorily prevent scattering of NaCl when used for a long time, since the same phenomenon as described above occurs.
Accordingly, an object of the present invention is to provide an air cleaning filter which does not show great increase in pressure loss even when the outside air is highly humid and the salt particles gathered on the filter surfaces deliquesce, and which in turn is free of passing of the deliquescent salt toward and out from a downstream filter.